Hypethetically, if one had the high-res PDFs of the cards, could one replace the low-res card images with those? For oneself only, of course.

You need *.pngs of the cards, but yes it's easy to make custom decks. You could take my mod, create a save of it--which you can manipulate however you want--and then create custom decks with high-res card images using the deck builder included with the game. You'll lose the scripting (because it's based on deck GUIDs and they'll be replaced if you replace the decks), but you could probably figure out how to reproduce that pretty easily.